On the definition of the excess permittivity of a fluid mixture. II

In a previous work, an ideal fluid mixture is assumed to be a homogeneous linear and isotropic system that verifies the following conditions: (I) no interaction between molecules, (II) point-like molecules and (III) under fixed temperature and pressure conditions the number of molecules per unit volume is the same for both the individual components and the mixture. It was shown that for such ideal mixture the permittivity is the sum of the individual permittivities weighted with the mole fractions of the components.In this work it is shown that independently of the latter assumption the permittivity of the system can be expressed as the sum of the individual permittivities weighted with the volume fractions of the components.     

On the definition of interfacial excesses in a system consisting of an insoluble solid adsorbent and a binary liquid mixture

The paper deals with the problem whether the interfacial excess volumeV ^σ has to be set zero in the case of the systems in question as stated by D. H. Everett, or eventually different from zero as proposed by G. Schay. First it is shown that the current operational definitions of the interfacial excess amounts as relative or reduced excesses are quite independent of the choice of the reference volume (or amount) of equilibrium bulk liquid. Everett operates with the Gibbs dividing surface, letting it coincide with the surface of the solid, the corresponding operational definition of the interfacial excess amount being then that ofn ^σ(v). It is shown that this definition, while compatible with those of the relative or reduced excesses, is not cogent. Relative and reduced excesses are usually interpreted as the values resulting when the adsorption of either one selected component or the total adsorption is set arbitrarily zero. With this interpretation, the assumptionV ^σ =0 for the total system could be maintained only by admitting an adsorption excess of the solid different from zero which appears, however, unnatural.     

Phase behavior of a symmetrical binary fluid mixture

We have investigated the phase behavior of a symmetrical binary fluid mixture for the situation where the chemical potentials mu(1) and mu(2) of the two species differ. Attention is focused on the set of interparticle interaction strengths for which, when mu(1)=mu(2), the phase diagram exhibits both a liquid-vapor critical point and a tricritical point. The corresponding phase behavior for the case mu(1) not equalmu(2) is investigated via integral-equation theory calculations within the mean spherical approximation and grand canonical Monte Carlo (GCMC) simulations. We find that two possible subtypes of phase behavior can occur, these being distinguished by the relationship between the triple lines in the full phase diagram in the space of temperature, density, and concentration. We present the detailed form of the phase diagram for both subtypes and compare with the results from GCMC simulations, finding good overall agreement. The scenario via which one subtype evolves into the other is also studied, revealing interesting features.     

On the use of computer simulation to determine the excess free energy in fluid mixtures

Haile, J.M., 1986. On the use of computer simulation to determine the excess free energy in fluid mixtures. Fluid Phase Equilibria, 26: 103–127This paper first reviews the use of Kirkwood's coupling parameter for determining residual chemical potentials, activity coefficients, and Henry's constants from Monte Carlo and molecular dynamics computer simulations. A new version of the method is then developed for obtaining the excess Gibbs free energy from isothermal-isobaric simulations. New expressions are also given for the excess volume and excess entropy.The revised method is demonstrated by computing excess free energies for 14 mixtures of repulsive soft spheres. Isothermal-isobaric molecular dynamics was used to generate the necessary simulation data. Although the excess free energies for these particular mixtures are small in magnitude (|G^E/NkT| < 0.1), the simulation method generally gives G^E within 5% of the values calculated by thermodynamic perturbation theory.     

On the definition of the atomic charge in a molecule

Atomic charges calculated by various methods for model two-center molecules have been compared with corresponding electron-count functions. Varying the overlap and polarity conditions in the model system, it could be found out under what conditions the individual definitions fail to predict physically reasonable values of atomic charges.     

The excess enthalpy for a mixture of krypton and xenon: experiment and theory

Measurements of the excess enthalpy of krypton and xenon mixtures at 163 K are reported. These results are found to disagree with the only other published result. This discrepancy is discussed. Conformal solution theory is used to provide an unbiased prediction of the excess enthalpy. We review published experimental and calculated excess thermodynamic properties of the K_r/Xe system at zero pressure and 161.38 K and 165 K.     

On the analyses of mixture vapor pressure data: the hydrogen peroxide/water system and its excess thermodynamic functions

Reported here are some aspects of the analysis of mixture vapor pressure data using the model-free Redlich-Kister approach that have heretofore not been recognized. These are that the pure vapor pressure of one or more components and the average temperature of the complex apparatuses used in such studies can be obtained from the mixture vapor pressures. The findings reported here raise questions regarding current and past approaches for analyses of mixture vapor pressure data. As a test case for this analysis approach the H2O2-H2O mixture vapor pressure measurements reported by Scatchard, Kavanagh, and Tickner (G. Scatchard, G. M. Kavanagh, L. B. Ticknor, J. Am. Chem. Soc. 1952, 74, 3715-3720; G. M. Kavanagh, PhD. Thesis, Massachusetts Institute of Technology (USA), 1949) have been used; there is significant recent interest in this system. It was found that the original data is fit far better with a four-parameter Redlich-Kister excess energy expansion with inclusion of the pure hydrogen peroxide vapor pressure and the temperature as parameters. Comparisons of the present results with the previous analyses of this suite of data exhibit significant deviations. A precedent for consideration of iteration of temperature exists from the little-known work of Uchida, Ogawa, and Yamaguchi (S. Uchida, S. Ogawa, M. Yamaguchi, Japan Sci. Eng. Sci. 1950, 1, 41-49) who observed significant variations of temperature from place to place within a carefully insulated apparatus of the type traditionally used in mixture vapor pressure measurements. For hydrogen peroxide, new critical constants and vapor pressure-temperature equations needed in the analysis approach described above have been derived. Also temperature functions for the four Redlich-Kister parameters were derived, that allowed calculations of the excess Gibbs energy, excess entropy, and excess enthalpy whose values at various temperatures indicate the complexity of H2O2-H2O mixtures not evident in the original analyses of this suite of experimental results.     

Dynamical structure factors for a fluid binary mixture in the hydrodynamic limit

Abstract

Using the correlation functions obtained by Cohen et al, the expressions for the number concentration dynamical structure factors for a binary alloy are given in the hydrodynamic limit. Kubo relations are derived and presented via some new structure factors, which, although not directly connected to the scattering in the mixture, are linearly related to the number-concentration structure factors. The second moments for the various structure factors are also given. Finally, sound attenuation in binary mixtures is briefly discussed.     

Experimental excess molar volumes of the ternary mixture and comparisonwith several empirical methods

Experimental excess molar volumes for the ternary system {x₁MTBE+x₂1-propanol+(1–x₁–x₂)nonane} and the three involved binary mixtures have been determined at 298.15 K and atmospheric pressure. Excess molar volumes were determined from the densities of the pure liquids and mixtures, using a DMA 4500 Anton Paar densimeter. The ternary mixture shows maximum values around the binary mixture MTBE+nonane and minimum values for the mixture MTBE+propanol. The ternary contribution to the excess molar volume is negative, with the exception of a range located around the rich compositions of 1-propanol. Several empirical equations predicting ternary mixture properties from experimental binary mixtures have been applied.     

On the density matrix definition of valency

In a recent article Gopinathan and Jug have proposed a definition of atomic valency which had previously been given by Armstrong, Perkins and Stewart for closed shell molecules. The validity and interpretation of this definition for open shell systems is discussed. A new parameter for structural analysis, the free electron index, is presented.We want to acknowledge the computer time made available by Centro de Estudios Superiores para el Procesamiento de la Informatión (CESPI) de la Universidad National de La Plata and many useful suggestions made by the referees of this paper.     

On the Dewar definition of resonance energy

Dewar's approach to the resonance energy concept is analysed and it is shown that the change in the reference structure is crucial condition for the theory to give an accurate prediction of aromaticity. Thus, the use of simple Hückel theory is justified. Besides, it is shown that part of the correlation energy is contained in the Hückel theory through the parameters α and β.     

On the definition of a Monte Carlo model for binary crystal growth

We show that consistency of the transition probabilities in a lattice Monte Carlo (MC) model for binary crystal growth with the thermodynamic properties of a system does not guarantee the MC simulations near equilibrium to be in agreement with the thermodynamic equilibrium phase diagram for that system. The deviations remain small for systems with small bond energies, but they can increase significantly for systems with large melting entropy, typical for molecular systems. These deviations are attributed to the surface kinetics, which is responsible for a metastable zone below the liquidus line where no growth occurs, even in the absence of a 2D nucleation barrier. Here we propose an extension of the MC model that introduces a freedom of choice in the transition probabilities while staying within the thermodynamic constraints. This freedom can be used to eliminate the discrepancy between the MC simulations and the thermodynamic equilibrium phase diagram. Agreement is achieved for that choice of the transition probabilities yielding the fastest decrease of the free energy (i.e., largest growth rate) of the system at a temperature slightly below the equilibrium temperature. An analytical model is developed, which reproduces quite well the MC results, enabling a straightforward determination of the optimal set of transition probabilities. Application of both the MC and analytical model to conditions well away from equilibrium, giving rise to kinetic phase diagrams, shows that the effect of kinetics on segregation is even stronger than that predicted by previous models.     

Liquid densities of THF and excess volumes for the mixture with water in a wide temperature and pressure range

In this paper densities for THF (tetrahydrofuran) and THF + water mixtures measured with the help of the Anton Paar DMA HPM vibrating tube densimeter are reported. The pure component densities of tetrahydrofuran measured in the temperature range from 278 to 437 K and pressures up to 130 MPa were correlated with the TRIDEN-System. Additionally densities of the binary mixture tetrahydrofuran + water were measured for 6 different concentrations in a temperature range from 288 to 338 K and up to 130 MPa. Excess volumes (vE)$(v^E)$ of the mixture were determined using the own correlation of the tetrahydrofuran densities and the equation of state (EoS) for water by Wagner and Pruß. Redlich–Kister polynomials were used to fit the vE-data$v^E\text{-data}$. Additionally in this work it was checked if the vibrating tube densimeter allows the determination of the miscibility gap for the system THF–water as a function of temperature and pressure.     

DISQUAC predictions of excess enthalpies of the ternary mixture: tetrahydrofuran + cyclohexane + butanenitrile

Excess molar enthalpies of the ternary mixture: tetrahydrofuran + cyclohexane + butanenitrile and of the binary mixtures involved have been measured at 298.15 K by means of a flow microcalorimeter.The DISQUAC analysis is developed using interchange coefficients previously determined for the binary mixtures. Even neglecting ternary interactions, it is possible to obtain a satisfactory representation of the ternary system.     

On the concept of critical surface excess of micellization

The critical surface excess of micellization (CSEM) should be regarded as the critical condition for micellization of ionic surfactants instead of the critical micelle concentration (CMC). There is a correspondence between the surface excesses Γ of anionic, cationic, and zwitterionic surfactants at their CMCs, which would be the CSEM values, and the critical association distance for ionic pair association calculated using Bjerrum's correlation. Further support to this concept is given by an accurate method for the prediction of the relative binding of alkali cations onto dodecylsulfate (NaDS) micelles. This method uses a relative binding strength parameter calculated from the values of surface excess Γ at the CMC of the alkali dodecylsulfates. This links both the binding of a given cation onto micelles and the onset for micellization of its surfactant salt. The CSEM concept implies that micelles form at the air-water interface unless another surface with greater affinity for micelles exists. The process would start when surfactant monomers are close enough to each other for ionic pairing with counterions and the subsequent assembly of these pairs becomes unavoidable. This would explain why the surface excess Γ values of different surfactants are more similar than their CMCs: the latter are just the bulk phase concentrations in equilibrium with chemicals with different hydrophobicity. An intriguing implication is that CSEM values may be used to calculate the actual critical distances of ionic pair formation for different cations, replacing Bjerrum's estimates, which only discriminate by the magnitude of the charge.